1. Field of the Invention
The present invention relates to electrooptical devices, substrates for driving the electrooptical devices and methods for making the electrooptical devices and the substrates. In particular, the present invention relates to a configuration having an active region of a bottom-gate-type thin-film insulating-gate field-effect transistor (hereinafter referred to as bottom-gate-type MOSTFT) using a single-crystal silicon layer, grown by heteroepitaxy on an insulating substrate, and a passive region. Herein, the bottom-gate types include an inverted NSI stagger type and an inverted ISI type. This configuration is suitable for liquid crystal displays etc.
2. Description of the Related Art
Various types of active-matrix liquid crystal displays are known. For example, a liquid crystal display has a display region using amorphous silicon for TFTs and ICs for external driving circuits. Another type of liquid crystal display integrates a display section using solid phase deposition polycrystalline silicon TFTs and driving circuits, as disclosed in Japanese Patent Application Laid-Open No. 6-242433. Integration of a display section using excimer laser annealing polycrystalline silicon TFTs and driving circuits is also known in Japanese Patent Application Laid-Open No. 7-131030.
Although conventional amorphous silicon TFTs have high productivity, they are not suitable for production of p-channel MOSTFTs (hereinafter referred to as pMOSTFTs) due to a low electron mobility of 0.5 to 1.0 cm2/v·sec. Since a peripheral driving section using pMOSTFTs and a display section cannot be formed on the same substrate, the driver IC should be an external component, which is mounted by, for example, a tape automated bonding (TAB) method, which has high production costs. This configuration inhibits production of high-resolution devices. Furthermore, the small electron mobility, as described above, causes a small ON current; hence, the size of the transistors in the display section is inevitably large, resulting in a small aperture ratio of pixels.
Conventional polycrystalline silicon TFTs have an electron mobility of 70 to 100 cm2/v·sec and can facilitate production of high-resolution devices. Thus, liquid crystal displays (LCDs) which use polycrystalline silicon and are integrated with driving circuits have attracted attention. The above electron mobility, however, is insufficient for driving a large LCD of 15 inches or more, and thus ICs for an external driving circuit are required.
TFTs using polycrystalline silicon formed by a solid-phase deposition process require annealing at a temperature of 600° C. or more for several tens of hours and thermal oxidation at approximately 1,000° C. to from a gate SiO2 layer. Thus, the production of such TFTs requires using a semiconductor production system. Thus, the wafer size is limited to 8 to 12 inches and the use of expensive heat-resistant quartz glass is inevitable, resulting in high production cost. Thus, the use of such TFTs is limited to electronic viewfinders (EVF) and audiovisual (AV) projectors.
Polycrystalline silicon TFTs produced by excimer laser annealing have many problems, including unstable output of the excimer lasers, low productivity, increasing price of the apparatus with increasing size, low yield and low quality. These problems are pronounced when large glass substrates having a side of, for example, 1 meter are used.